Lecture16.md

Lecture 16

Accessors/Mutators, System Modelling + Relationships

Last time

Keep fields private!

Provide aggressors (getters) and/or mutators (setters) which are public.

class Vec {
  int x,y;
  public:
  int getX() const {return x;}
  int getY() const {return y;}
  void setX(int n) {x = n;}
  void setY(int n) {y = n;}
};

Suppose a class has private fields and no aggressors/mutators.

• Want to implement the I/O operators
  • as standalone functions

class Vec {
	int x,y;
  friend std::ostream & operator<<(std::ostream &, const Vec &);
};

System Modelling

• Identify the abstractions/entities/classes
• Interaction/relationship between classes

UML - Unified Modelling Language

• Scratching the surface

• Whenever possible use general types not specific to a language

Composition

class Vec {
	int x,y;
	public:
	Vec(int, int);
};

class Basis {
	Vec v1,v2;
};

Basis b; // won't compile as cannot default construct v1,v2

Solution

1. Provide default constructor for Vec.
2. Use the MIL to call on alternate constructor

Basis::Basis() : v1{0,1}, v2{1,0} {}
Basis b; // Valid; never default constructs v1,v2

• Embedding an object within another is composition.
• When an object takes ownership of another object.

Composition creates an OWNS-A relation

• Basis OWNS-A Vec

Typically, A OWNS-A B, if:

• When A is copied, B is copied
• When A is destroyed, B is destroyed

// List OWNS-A Node
class List {
	...;
  Node * theList : ... {}
};

// Car OWNS-A Wheels
class Car {
	Wheel w1,w2,w3,w4;
};

// Is it still OWNS-A?
class Car {
  wheel *w1, *w2, *w3, *w4;
};

Aggregation

Parts in a Catalogue: Catalogue HAS-A Car-Part

Typically, A HAS-A B, if:

• When A is copied, B is not
• When A is destroyed, B is not

class Lake {
	Goose **geese;
};

Inheritance

Unable to create a collection that contains a mix of Book/Text/Comic.

Observation:

• Text IS_A Book (with an extra topic field)
• Comic IS_A Book (with an extra hero field)

Derived class inherits ALL (public & private) members (fields as methods) from the Base class.

Any method you can call on a Book object, you can call on a Text object.

• ifstream IS_A istream

Text inherits private fields: title, author, numPages

• Yet Text objects cannot access these fields.

Text::Text(string t, string a, int n, string topic) :
	title{t}, author{a}, numPages{n}, topic{topic} {}

Won't compile:

1. Inherited fields are also private.
2. MIL can only refer to fields declared in the class.
3. No default constructor for Book.